Is there any way to connect 2 computers together similar to the good old FIREWIRE and transfer your data? maybe that USB-C on that new motherboard may work?
so I use the router as a gateway or a direct connection? and if I need the ethernet how to make each other work as a connected USB/pen drive/external HDD?Yes, it's called Ethernet.
If you want to do it over USB, you need to get a USB data transfer cable.